Sepsis is a major cause of morbidity and mortality in both adults and children with an estimated 750,000 cases per year in the United States. To improve outcomes in these patients, it is vital to increase our understanding of the pathophysiology of the disease and identify new therapeutic targets. Sepsis occurs when the tightly con- trolled host response to infection extends beyond the local environment and into the systemic circulation. This results in complex interactions involving microbes, blood cells, and the endothelial barrier in the microcircula- tion that can progress to vascular collapse and organ failure. Neutrophils are key early responders to infection. Neutrophils kill microbes by phagocytosis and by oxidant-mediated microbe killing in the surrounding environ- ment. Myeloperoxidase (MPO) is a major mediator of oxygen-dependent microbicidal activity. MPO catalyzes the conversion of H2O2 to the potent oxidant, hypochlorous acid (HOCl), which reacts with both microbial and host molecular targets including proteins and lipids. The Ford group at Saint Louis University (SLU) has used physiological, biochemical and bio-organic approaches to demonstrate that the vinyl ether bond at the sn-1 po- sition of plasmalogen lipids is targeted by HOCl, resulting in the production of 2-chlorofatty aldehyde, which is metabolized to 2-chlorofatty acid under physiological and pathophysiological conditions. These chlorinated li- pids are elevated in activated neutrophils as well as in in vivo inflammation models including LPS treatment, Sindai virus exposure and peritonitis. Pilot studies show elevated levels of chlorinated lipids in a rat model of sepsis, and in human plasma of sepsis patients compared to controls. Additional preliminary data provide strong evidence that chlorinated lipids modulate leukocyte, platelet and endothelial cell function in the microcir- culation and in isolated cell systems. Accordingly, we have assembled a multidisciplinary group with three PIs to test our hypothesis that chlorinated lipids produced by activated leukocytes during sepsis are mediators of severe endothelial dysfunction resulting in multiple organ failure. This hypothesis will be tested by three specific aims. Specific Aim 1 will test the hypothesis that chlorinated lipids produced by neutrophils are key mediators of endothelial dysfunction and organ damage during sepsis. Specific Aim 2 will test the hypothesis that chlorinated lipids mediate dysfunction in human endothelial cells. Specific Aim 3 will test the hypothesis that plasma 2-ClFA levels indicate both severity and therapeutic effectiveness in sepsis pa- tients. Overall, a multi-disciplinary approach will examine the role of chlorinated lipids produced as a result of leukocyte activation during sepsis as indicators of the severity of human sepsis, and mediators of vascular en- dothelial dysfunction examined both in vivo in the rat microcirculation and in vitro for mechanistic insights. The project is innovative and significant. These studies are designed to discover new paradigms for the role of neutrophils in eliciting endothelial dysfunction providing new targets for therapeutics to treat septic patients.